Apparatus and methods for mass production of universal or DC electric motors have been developing for many years. Initially, soft soldering or brazing was employed to join the armature wires to the commutators. However, the need for high-quality motors which are reliable and economical has resulted in the growth and increased utilization of commutator brazing and fusing to produce such motors. In addition, in order to further improve production methods, tang-type commutators were developed, as compared to the prior slotted-type commutators, which made it simpler for attaching the armature lead wires to the commutator bars. An example of such apparatus which discloses efficient and rapid fusing of both the tang-type and slotted-type of commutators, is shown in U.S. Pat. No. 3,045,103. However, as the need has increased for extremely high-quality control and uniform commutator joints, when produced on a mass scale, there has been a desire to improve such apparatus.
The need for such careful quality control arises from the nature of commutator fusing and brazing itself. As distinguished from spot-welding processes, in commutator fusing techniques, the heating electrodes generally employed to heat the armature wires and commutator bars are high-resistance electrodes, whereas in commutator brazing, high- or low-resistance electrodes may be employed. In addition, brazing employs high-temperature solder alloys. As will be understood, the parts to form the joint are heated by the heat dissipated by these electrodes. In addition, since the armature lead wires are normally insulated, sufficient current must be applied to the electrodes so that they can be heated to a temperature sufficient to remove the insulation, but this must be carried out within a relatively short period of time so as not to damage the armature lead wires themselves. It can therefore be seen that the amount of heat applied to the joint must be controlled with a high degree of accuracy.
U.S. Pat. No. 4,079,225 discloses an arrangement for controlling brazing and fusing operations for producing high-quality and uniform joints. Although the apparatus disclosed therein has been particularly well suited for fusing operations, it has not been completely satisfactory for brazing operations. More particularly, in the arrangement disclosed therein, when a peak predetermined temperature has been reached, current to the electrodes is turned off, at which point the brazing or fusing operation takes place while the commutator bar and armature wires are cooling.
This has been generally satisfactory for fusing operations, since sufficient energy remains within the workpiece. However, in brazing operations, a brazing alloy must be applied to form the joint, and the temperature level drops below the required brazing temperature, for example 1300.degree. F., before the brazing operation is completed. Accordingly, in order to maintain the workpiece at a temperature level sufficient for brazing, it has been necessary to overshoot the temperature level required for brazing by at least a few hundred degrees. For example, if the required brazing temperature is 1300.degree. F., the current to the electrodes is not shut off until the temperature of the workpiece reaches approximately 1700.degree. F. or 1800.degree. F. In this manner, during the short time interval that it takes the workpiece to cool down to the 1300.degree. F. level, the brazing operation can be performed. However, the disadvantage of overshooting the required brazing temperature level is that the commutator bar may be annealed, discolored, or disfigured, and in addition, the quality of the joint may be impaired at such high-temperature levels.
In view of the foregoing, there is a need for controlling brazing apparatus with a higher degree of accuracy so that the temperature level of the workpiece is maintained within a desired range while the joint is being formed without having to substantially overshoot the preferred temperature level for brazing.
Accordingly, it is an object of the present invention to provide a method and apparatus which overcomes the aforesaid problems. Specifically, it is within the contemplation of the present invention to provide an improved method and apparatus for controlling brazing operations with a high degree of accuracy to obtain high-quality and uniform joints by maintaining the heating means within a desired range to form the joint.